1. Field of the Invention
The present invention relates to a cylindrical telescopic kelly-bar apparatus for use with a large-diameter excavator such as an earth drill.
2. Description of the Related Art
Vertical excavation employing a large-diameter excavator, such as an earth drill, is performed as follows. Kelly-bars which are assembled in multiple stages are sequentially extended to make excavation while turning the excavating bucket connected to the free end of the excavator. When the excavating bucket is charged with earth and sand, the kelly-bars are sequentially contracted to lift the bucket above the ground, thereby discharging the earth and sand. A vertical hole is excavated by repeating the above-described process.
A known type of cylindrical three-stage kelly-bar apparatus will be explained below with reference to FIGS. 23-27. FIG. 23 shows the kelly bar apparatus extended to its full length in a right side cross section, while FIG. 24 shows the apparatus contracted to its minimum length in a partially omitted, magnified form. FIG. 25 shows the details of the bottom end portion of the kelly-bar apparatus, and FIG. 26 shows the details of the retaining mechanism used in the kelly bar apparatus. FIG. 27 shows the external appearance of an inner kelly bar.
In the illustrated cylindrical telescopic kelly-bar apparatus, an inner kelly-bar 21 is inserted into a second kelly-bar 22 which is in turn inserted into an outer kelly-bar 23. These kelly-bars are arranged to telescopically extend and contract and to transmit to the inner kelly-bar 21 rotational torque imparted to the outer kelly-bar 23 by a kelly driving device 37. More specifically, each of the inner, second and outer kelly-bars 21, 22 and 23 is provided with three torque transmitting bars 24. These three bars 24 extend throughout the corresponding kelly-bar and are spaced apart from one another around the outer periphery of the same. Rectangular grooves 25 for engagement with the respective bars 24 are formed in the bottom end portion of each of the second and outer kelly-bars 22 and 23. The portion of each of the cylindrical kelly-bars 22 and 23 which is provided with the rectangular grooves 25 for engagement with the respective bars 24 is called a torque transmitting portion TT. The bars 24 of the outer kelly-bar 23 are engaged with the kelly driving device 37 actuated by a hydraulic motor (not shown), whereby rotational torque is applied by the kelly driving apparatus 37.
As shown in FIGS. 26 and 27, the inner kelly-bar 21 has a cylindrical upper shaft 21a and a lower shaft 21b having a square shape in cross section, the lower shaft 21b being fitted into and welded to the bottom end of the upper shaft 21a. A brim-like spring support 30 is welded to the lower shaft 21b. A damping spring 31 is held on the spring support 30, and a ring-shaped spring support 32 is held on the top of the damping spring 31. Reference numeral 21c denotes a mounting bore 21c for connection to an excavating bucket (not shown).
The top end of the upper shaft 21a is connected to a suspending rope 34 by means of a swivel joint 33. Although not shown, the suspending rope 34 is suspended from one end of the boom of the large-diameter excavator, and the other end of the rope 34 is wound around the winch of the large-diameter excavator.
On the other hand, the lower shaft 21b of the inner kelly-bar 21 positioned at the bottom end thereof is inserted into the connecting rectangular opening of the excavating bucket 35. The lower shaft 21b and the excavating bucket 35 are connected to each other by a pin 36. As shown in FIG. 26, a strip-shaped retaining member 22a and a strip 22b for preventing radial oscillation are fixed to the top end of the second kelly-bar 22.
The operation of the kelly-bar apparatus having the above-described arrangement and construction will be explained below. In an initial stage of excavation, the inner kelly-bar 21, the second kelly-bar 22 and the outer kelly-bar 23 are in a contracted state and the bottom end faces of the respective kelly-bars are maintained in contact with the kelly-bar receiving plate 32 as shown in FIGS. 24 and 25. When the outer kelly-bar 23 is rotated about its axis by the kelly driving apparatus 37, the rotational torque of the outer kelly-bar 23 is transmitted to the second kelly-bar 22 and the inner kelly-bar 21 in sequence by the engagements between the bars 24 and the respective rectangular grooves 25. The inner kelly-bar 21 is in turn rotated about its axis to cause the excavating bucket 35 to make excavation. The above-described process proceeds as the suspending rope 34 is gradually moved down. Initially, the inner kelly-bar 21, the second kelly-bar 22 and the outer kelly-bar 23 are integrally moved down until a top retaining member 23b of the outer kelly-bar 23 falls on the kelly driving device 37. When excavation further proceeds, the inner and second kelly-bars 21 and 22 integrally extend from the outer kelly-bar 23. As shown in FIGS. 23 and 26, the ring-shaped retaining member 22a of the second kelly-bar 22 falls on the top end face of the torque transmitting portion TT of the outer kelly-bar 23, and only the inner kelly-bar 21 moves down to continue excavation.
The above described conventional kelly-bar apparatus has a number of problems as follows.
1) The second kelly-bar 22 and the outer kelly-bar 23 each has a cylindrical configuration. The inner kelly-bar 21 includes the cylindrical upper shaft 21a and the prism-shaped lower shaft 21b for the purpose of connecting the excavating bucket 35 with the bottom end of the inner kelly-bar 21. Accordingly, the cross-sectional configuration of the inner kelly-bar 21 abruptly changes from circle to rectangle at the portion where the upper shaft 21a is connected to the lower shaft 21b, and stress concentration easily occurs in such a cross-sectionally irregular portion. In the case of the conventional arrangement in which the cylindrical upper shaft 21a and the prism-shaped lower shaft 21b are, as described above, connected to each other by simple circumferential welding as shown by symbol WD1 in FIG. 25, the fatigue strength is low and the life of the kelly-bar apparatus is limited.
To overcome the above-described disadvantages, the following process may be considered. The lower shaft 21b is formed from a rectangular material by cutting, and the lower portion of the lower shaft 21b beneath the spring support 30 is formed as a prism-like portion for connection with a bucket. The upper portion of the same above the spring support 30 is formed as a cylindrical portion having the same diameter as the upper shaft 21a, and the top end of this cylindrical portion is welded to the upper shaft 21a. The use of this structure allows the welded portion and the cross-sectionally irregular portion to be offset from each other, thereby making it possible to relax stress concentration. However, this structure has the following problem. The aforesaid bars 24 extend throughout the length of the outer periphery of the cylindrical portion above the spring support 30, and the spring support 30 is fitted onto and welded to the bottom end of the cylindrical portion. If the bottom ends of the bars 24 are abutted on and welded to the top face of the spring support 30, stress concentration in the fillet welded portion of the spring support 30 is combined with stress concentration in the welded portion of the bottom end of the bars 24. If welding is performed with no bottom end of the bars 24 maintained in contact with the spring support 30, the fatigue strength is reduced.
In addition, in the conventional arrangement, although the spring support 30 is fitted onto the cylindrical portion or the lower shaft 21b of the prism-shaped portion by fillet welding as indicated by symbol WD2 in FIG. 25, impact occurring when the second or outer kelly-bar 22 or 23 has fallen is transmitted to the spring support 30 through the damping spring 31 and shearing forces directly act on the welded portion. This may lead to a reduction in the lifetime of a kelly-bar apparatus.
2) It is desirable that the second kelly-bar 22 be arranged in order to improve the strength thereof.
2-a) Since the bars 24 slide along the inner walls of the respective rectangular grooves 25 of the torque transmitting portion TT during the rotation of the kelly-bar apparatus, portions including the respective rectangular grooves 25 are exposed to wear when the apparatus is in use. If the wear excessively proceeds, a crack may be formed in the bottom end portion of the torque transmitting portion TT. If the crack proceeds upwardly and reaches the top of the torque transmitting portion TT, the open end 23a of the outer kelly-bar 23 is opened in flared form as shown by two-dot-dashed lines in FIG. 26. As a result, the ring-shaped retaining member 22a may not be able to engage with the top of the torque transmitting portion TT. For these reasons, when the above-described kelly-bar apparatus is in use an operator is forced to consistently inspect the state of wear on the rectangular grooves 25.
2-B) If torque is applied to one end of the kelly-bar which is fixed at the other end, a shearing strain occurs over the entire length of the kelly-bar. Accordingly, the central axes of the respective recesses 25 on the driven side of the kelly-bar are inclined with respect to the central axis of the fixed side of the kelly-bar, and the grooves 25 of the driven (outer) side of the kelly-bar and the bars 24 of the fixed (inner) side of the kelly-bar are inclined in the axial direction. As a result, the grooves 25 and the corresponding bars 24 come into local contact with each other near the edges of the respective grooves which are positioned at the top and bottom of the torque transmitting portion TT, thus leading to high surface pressure or high stress.
Since the kelly-bar is made from a material such as an elongated pipe or a round bar for its functional reason, it is easily bent in the direction perpendicular to the axis. The material itself also includes bending or strain. In addition, since a plurality of kelly-bars having different bendings or strains are combined by insertion, the inclination of the lengthwise central axis of the inner kelly-bar does not coincide with that of the lengthwise central axis of the outer kelly-bar, i.e., both central axis are relatively inclined. If sharp edges are formed at the ends of the respective recesses of the torque transmitting portion of the kelly-bar, the tips of the edges come into contact with the inner kelly-bar and high surface pressure and high stress locally occur. Also, in the inclination between the axes of the inner and outer kelly-bars, the bars 24 come into contact with the respective grooves 25 near the open end (bottom end) of the outer kelly-bar. Accordingly, the above-described structure is disadvantage in terms of strength. For this reason, it is necessary to excessively increase the outer diameter or wall thickness of the torque transmitting portion TT. Furthermore, if sharp edges are formed at the ends of the grooves of the torque transmitting portion TT, a specific kelly-bar is scratched by the sharp edges and smooth telescopic operation can not be achieved.